In its natural unmined state, lead contains a number of isotopes, including .sup.210 Pb. .sup.210 Pb is radioactive with a half-life of about 22 years. Therefore, "artesian lead," being centuries old, has a significantly lower concentration of .sup.210 Pb compared to natural lead that is relatively young.
The world's supply of artesian lead is disappearing, unfortunately, at the same time the demand for it is increasing. Many electronic applications require lead (particularly for solder) with an unusually low concentration of .sup.210 Pb. The radioactivity of .sup.210 Pb ultimately results in .alpha. particles that cause soft errors in electronic circuits.
Also, the electronic industry is moving to higher densities and three-dimensional architectures. Sensitivity to .alpha. particles increases with decreasing voltage, feature size and increasing numbers of interconnects and density.
Atomic vapor laser isotope separation (AVLIS) has been demonstrated on a number of elements, including uranium, gadolinium and other lanthanide elements. For example, Paisner et al., U.S. Pat. No. 5,202,005 (1993), teaches isotopic enrichment of .sup.157 Gd by selective photoionization.
However, whether AVLIS is technically feasible for a specific element typically requires detailed consideration of a wide variety of issues including the photo-pathway, spectroscopic preparation of the feed vapor stream and the efficient collection of the product free of contamination by either tailings or feed. A definitive evaluation of the applicability of AVLIS requires knowledge of the isotope shifts, hyperfine splittings, state energies and designations, state lifetimes and branching ratios. For those cases where AVLIS is technically feasible, an economic evaluation follows to determine the cost competitiveness of this technique versus the product value, for example.
These analyses require knowledge of the photo-pathway, knowledge of the number, powers, frequencies and other characteristics of the lasers, as well as knowledge of the material properties which lead to the determination of the vapor properties. Photo-pathways typically require excessive laser power and do not result in an economically realizable isotope separation process. Careful matching of existing lasers to possible transitions is also an economic consideration.
With Pb, for example, the F=3/2 hyperfine component of .sup.207 Pb is the nearest neighbor of .sup.210 Pb and has a natural abundance of about 23%. The isotope shift for a transition between .sup.210 Pb and .sup.207 Pb is about 1 GHz. The close frequency proximity of .sup.210 Pb and its nearest neighbor tends to give rise to stimulated Raman scattering (SRS).
A related problem is that .sup.207 Pb absorbs a substantial amount of 283 nm (doubled 556 nm) power that is expensive to generate.
Both problems are important because of the small isotope shift and the very small natural abundance of .sup.210 Pb, at most on the order of parts per billion.
Laser isotope separation of lead, if developed, can produce a reliable, cost-effective and very large (on the order of one to ten metric tons per annum) supply.